GX339-4 in the X-Ray High, Soft State

We propose Target of Opportunity (TOO) XTE+USA observations of the BHC GX339-4 in the X-ray high (soft) state. High-sensitivity broadband observations of this source have never been made in this state. The spectral data will determine physical processes driving the emission, which we will contrast with our recent studies of the simultaneous Ginga+OSSE observations of GX339-4 and Cyg X-1 in the X-ray low (hard) state. We propose two TOOs, each one observing day (i.e. about 55ksec livetime) in duration, under separate trigger criteria specified in the text.